Art--It turns out that you don't need to know anything about harmonies, or even be able to harmonize to sing harmonize with others-- you can just sing the melody!

Seriously though, when you play and sing a soloist, you have to do all sorts of things to fill up the spaces--

When you work with other musicians and singers, you have to thin down what you are doing considerably--

One thing that is hard for me is to sing without having an instrument in my hands--and I also follow my own instrument for pitch--I was at a party with some guys who had sung with a harmony group from the 60's--they did let me sing the melody--but without my guitar, I suffered from pitch creep, and they kept waving their hands "higher" and making me start over again!